The following prior art was considered in preparing the patent application: U.S. Pat. No. 3,707,475; U.S. Pat. No. 3,560,514; U.S. Pat. No. 3,558,644; J. G. Lombardino and E. H. Wiseman, J. Med. Chem. 17:1182 (1974) and E. H. Wiseman, H. M. McIllhenny and J. W. Bettes, J. Pharm. Sci. 64:1469 (1975).